In a vehicle provided with a torque converter having a lock-up clutch, there is known a so-called start slip control, in which a torque transmission efficiency is improved by performing a slip control for the lock-up clutch during a vehicle start in order to improve fuel efficiency.
During the start slip control, just by performing a feedback control based on a proportional/integral control in an instruction pressure control of the lock-up clutch of the torque converter, a deviation from a target slippage disadvantageously increases when a slippage increases in response to a change of the engine torque during acceleration/deceleration.
For this problem, in JP 4-203561A, JP 2006-46636 A, and JP 2011-122619 A, a technique has been discussed, in which the instruction pressure of the lock-up clutch is controlled based on a lock-up differential pressure value subjected to a feedforward control and a feedback control value obtained from the engine torque signal estimated from an engine operation state as well as the feedback control value.